The most usual manner of controlling commutation from one phase to another in reluctance motors, particularly so-called switched-type drive motors, is to use at least one rotor sensor which is adapted to produce control signals that correspond to the rotational position of the rotor in relation to the stator. This is referred to below as the rotor position. This means, however, that an additional element must be mounted on the rotor, which in practice has been found to encumber a number of drawbacks and to give rise to errors, particularly in troublesome environments.
Brushless d.c. motors which operate with variable reluctance are well known to the art. The type of reluctance motor for which the invention is intended comprises a stator on which one or more excitation windings are arranged in one or more phases, with separate energizing or activation of the winding belonging to the respective phases. Both the stator and the rotor are normally provided with pronounced poles or teeth. The rotor has no winding. The stator and the rotor form a magnetic circuit for generating a mechanical torque which is substantially proportional to the square of the magnetomotive force of the excited or activated winding and to the permeance change time, which is a function of the movement of the rotor in the motor. Movement of the rotor relative to the stator generates a variation in the reluctance and therewith the permeance in the magnetic circuit of the stator winding.
Torque is only obtained in the drive direction of the motor when the magnetomotive force of the winding is maintained for a rotor position period in which the permeance increases with rotor movement. Consequently, it is desirable to hold each winding energized solely during one such period for this winding. For reasons pertaining to drive techniques it is convenient, although not fully necessary, to energize or activate solely one stator phase at a time, i.e. with no overlap between the phase energizations. Commutation from one phase to another can be effected so that each phase winding is energized or activated during a rotor position period in which the permeance increases with rotor position changes. The supply to each phase winding should be discontinued, or decreased, during each rotor position period in which the permeance decreases with rotor position change. As beforementioned, the most usual method in this regard is to use additional rotor sensors for sensing continuously the position of the rotor and controlling the energizing or power supply with the aid of a sensor-controlled circuit. There is a general desire, however, to find ways and means which will allow these rotor position sensors to be dispensed with. Consequently, several attempts have been made to utilize the variation of the current or voltage characteristics of the stator windings and surrounding circuits to provide an indication of suitable points of time at which the supply voltage can be switched on and off.